1. Field of the Invention
The present invention relates to piezoelectric elements, and methods for manufacturing the same.
2. Description of the Related Art
In general, piezoelectric elements have a structure in which a piezoelectric composed of inorganic oxide is interposed between two electrodes. The piezoelectric element is capable of electromechanical conversion in which deformation is generated in the piezoelectric by applying an electric filed across the electrodes. The piezoelectric expands and contracts according to electrical signals applied through the electrodes. Polycrystal sintered materials such as lead zirconate titanate (PZT) may be representative as the piezoelectric materials.
A piezoelectric body may be damaged by repeated extension and contraction operations. For example, in the case of a piezoelectric actuator described in JP-A-2007-006620 having a structure in which a part of a piezoelectric layer is interposed between two electrodes, the single piezoelectric layer has a portion that is operated and another portion that is not operated. The potion of the piezoelectric layer that does not expand or contract may be damaged due to stress caused by expansion and contraction of the portion that expands and contracts. In particular, in the case where the lower electrode is disposed on the inner side more inside than the outer periphery of the bottom portion of the piezoelectric layer, the size of crystal grains (grain size) of the sintered body of piezoelectric material formed on the lower electrode is generally different from the grain size of piezoelectric body that is not formed on the lower electrode.
According to the study conducted by the inventor, it is found that smaller grains are formed in a portion of a piezoelectric layer that is formed on an electrode, and larger grains are formed in a portion of a piezoelectric layer that is not formed on an electrode. Therefore, in the above-described example, larger grains are formed in the portion where the piezoelectric layer does not operate, such that stress would likely concentrate at interfaces of the grains in this non-operating portion, which lowers the mechanical strength and makes the layer more susceptible to damage.